Determining the size of an Earth-size exoplanet by the amount of starlight it blocks hundreds of light-years away once was the realm of science fiction. Measuring the mass of such a small planet based on its gravity was another level entirely, but astronomers have done just that for an exoplanet fifty percent the size of Earth.
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| The artistic concept shows the planetary system harboring Kepler-138b, the first exoplanet smaller than Earth with both a mass and size measurement. The sizes of the planets relative to the star have been exaggerated. Source: NASA |
To determine a planet’s mass, astronomers typically measure the minuscule movement of the star caused by the gravitational tug of an orbiting planet. For planets the mass of Earth detecting such a tiny tug is extraordinarily challenging with current technology. Fortunately, when a star hosts multiple planets that orbit closely together, scientists have developed another way to get at the planets’ masses.
Daniel Jontof-Hutter, a research associate at the Pennsylvania State University’s Center for Exoplanets and Habitable Worlds, led a team of astronomers in a study to measure the mass of all three planets by precisely observing the times each planet passed in front of, or transited, the star Kepler-138.
“Each planet periodically slows down and accelerates ever so slightly from the gravity of its neighboring planets. The slight change in time between transits allows us to measure the masses of the planets,” said Jontof-Hutter.
Each time a planet transits a star it blocks a small fraction of the star’s light, allowing astronomers to measure the size of the planet. This is how the Kepler spacecraft has detected thousands of planets around other stars.
By measuring both the mass and size of an exoplanet, scientists can calculate the density and infer the bulk composition to determine if a planet is predominantly made of rock, water or gas. Tiny Kepler-138b’s density is consistent with a rocky composition like Earth or Mars, but further observations are needed before astronomers can definitively say that it is a rocky world.
Kepler-138b is the innermost of three planets that orbit Kepler-138, a star less than half the size of our sun and roughly 30 percent cooler. The Kepler-138 system is located about 200 light-years from Earth in the direction of the constellation Lyra.
The outer two planets, Kepler-138c and Kepler-138d, are approximately the size of Earth. Kepler-138c is likely to be rocky, whereas Kepler-138d is less dense and cannot be made of the same mix of material as Earth. All three planets orbit too close to their star for liquid water to exist on the surface and support life, as we know it.
